Door systems are used in particular in rail and road vehicles, but also in boats and on aeroplanes. Such door systems are monitored in many respects and controlled depending on a large variety of conditions. On the one hand, the safety of boarding and de-boarding passengers must always be safeguarded, on the other hand, doors and door wings must open and close reliably and only under certain circumstances. It must also be prevented that persons or objects are trapped between two door wings or between a door wing and a door frame.
In addition, the process of boarding and de-boarding is often delayed when passengers or objects are blocking the opening and closing of doors.
In accordance with the disclosure, the term door systems also includes boarding aids such as sliding treads and tread steps. Their function must also be monitored and controlled, since they may hold obstacles posing a danger for passengers. It must also be ensured that no persons or objects are trapped between a boarding aid and the platform and that the boarding aid does not bump against persons.
For these numerous monitoring and control tasks, a variety of sensor means are used in door systems, such as:                1. Various push buttons for operating the door.        2. Push bars/switch rails for recognizing obstacles. These are mostly installed in a main closing edge, either alone or also additionally in secondary closing edges inside the doors or door wings.        3. Light barriers inside the vehicle to hold open doors being used (for controlling automatically closing doors).        4. Step sensors (safety mats, strain gauges or such) to recognize loads on treads and boarding aids.        5. Power bars on step systems to recognize collisions with the platform or with passengers when steps are being extended.        6. Ultrasound sensors in step systems for measuring the distance to the platform and also the platform height.        
The above named sensor systems require numerous components, sophisticated cabling and careful maintenance and upkeep. It is therefore necessary but also relatively expensive to completely monitor and satisfactorily control a door system.